1. Field of the Invention
The present invention relates to abnormal state detecting circuits of an inverter, and more specifically to an abnormal state detecting circuit of an inverter wherein abnormal state can be rapidly detected in open-phase state or simultaneous ignition of upper and lower arms.
2. Description of the Prior Art
FIG. 1 shows an abnormal state detecting circuit of an inverter in the prior art. In FIG. 1, reference numeral 1 designates DC power source. SCR1 and SCR2, and SCR3 and SCR4 designate thyristors to constitute pairs of arms respectively. A circulation diode and a transfer circuit are unnecessary for the description and therefore shall be omitted. Numeral 2 designates an inverter transformer, and a current transformer 3 is installed on the primary winding TR1 of the inverter transformer 2. A resistor 4 is connected in parallel to the output end of the secondary winding of the current transformer 3. Voltage drop of the resistor 4 is compared with a reference voltage 5 by a comparator 6.
Operation of the detecting circuit in the prior art will now be described. If current i.sub.b flowing through closed circuit of DC power source 1-TR1-SCR2-DC power source 1 in FIG. 1 becomes overcurrent by any reason, current flowing through the current transformer 3 increases and the voltage drop in the resistor 4 increases and becomes more than the reference voltage 5. Then the comparator 6 outputs an overcurrent signal 7 of the inverter transformer primary winding. Thus in the prior art, an abnormal state cannot be detected if the overcurrent is not generated as a result of fault in the inverter. In this constitution, when the inverter transformer is partially excited by ignition failure of one arm, i.e. fault of open-phase mode and therefore time is required before the overcurrent flows, the detection may be delayed. Furthermore, when a pair of arms connected between positive and negative poles of the DC power source are subjected to simultaneous open-phase state or simultaneous ignition, the detection cannot be performed at all.